Present day disk drives employ a number of servo techniques for controlling transducer positioning. Larger form-factor disk drives employ a dedicated servo surface on one disk platter to enable the generation of signals for servo control of read/write head position. In such larger disk drives, this servo signal generation technique is practical since most of those drives contain multiple platters, yielding many recording surfaces. The dedication of one surface to servo signals subjects the disk drive to a minimal "servo overhead".
Smaller form-factor disk drives (e.g., 3.5 inches or smaller) generally contain only one or two disks. In such a drive, if one surface of a disk is dedicated solely to servo information, the overhead can be as high as 50%--which is unacceptable. To minimize servo overhead in the smaller disk drives, servo information has been embedded into the data tracks and is distributed throughout each track. Since servo and data fields are thereby contiguous, differentiation between them becomes critical to the proper operation of the disk drive.
Before a disk drive can be operated to store data, the disks must be formatted. Such formatting involves the partitioning of each recording surface into a plurality of pie-shaped sectors, each sector including a single servo burst. Since each disk track has a different linear velocity from every other track on the disk, signals written at a constant frequency do not exhibit a constant data density from track to track. For instance, a servo burst written at a constant frequency in a sector of an inner track will occupy less linear distance on the inner track than a constant frequency servo burst written on a more radially distant outer track. Nevertheless, servo bursts are written using constant frequency signals across the tracks. In such a case, radially aligned servo bursts occupy different linear distances in their respective tracks, but radially aligned servo bursts occupy an identical time window in each track (the angular velocity of each track is identical). Thus, a timing signal generated to window a servo burst and various signal segments therein can be made the same for each track, irrespective of the radial distance of the track from a disk's spindle axis.
If constant frequency recording is used for data, as it is for servo bursts, the density of data in outer tracks is much less than for data in inner tracks and the resultant storage efficiency of the disk drive suffers. Many hard-disk disk drives therefore employ a method of recording termed "constant density" recording. More specifically, data is written in each track (or in a zone of tracks) so that the signal density is constant across all tracks in a zone. As a result, the frequency of recording of such data in greater radius tracks is higher than the frequency of recording in the more radially-inner tracks.
The prior art has employed two techniques to implement constant density recording. The first method varies the rotational speed of the disk and senses data at a constant data rate. The second technique maintains the rotational speed of the disk constant while increasing the recording rate as tracks approach the outer edge of a disk. The first method has been used in floppy disk drives, but in hard disk drives, it is difficult to vary the disk rotation speed. Therefore, hard disk drives typically use (as aforestated) different recording rates for different zones of tracks, depending upon the location of the track on the disk.
Prior art disk drives using embedded servo bursts have generally appended only a single data field to the servo burst. By knowing the duration of the servo burst, the position of a data field can be determined and the read/write circuitry properly enabled. A problem with the single data field/servo burst configuration is that the recording density on outer tracks suffers if constant size data blocks are employed.
Accordingly, it is an object of this invention to provide a disk drive employing embedded servo bursts with an ability to commence a data sector at any point within the data portion of a disk track.
It is another object of this invention to provide a disk drive employing embedded servo bursts with an ability to split data fields so that they appear in different disk sectors.
It is yet another object of this invention to provide a disk drive employing embedded servo bursts, with circuitry that generates start of data sector pulses at the commencement of each data sector, irrespective of whether a data sector is complete or split.